1. Field of the Invention
This invention relates to a coating method, and more particularly to a coating method of applying a coating liquid uniformly onto both sides of a web.
2. Description of Related Art
Various kinds of coating methods such as a roll coating method, a gravure coating method, a slide bead coating method and a doctor coating method have been conventionally used as a method of applying a coating liquid onto a web. However, an extrusion coating method has recently come to public attention as it provides a high productivity and a high operating performance, as well as an outstanding controllability of thickness of a coating film on the web.
In the extrusion coating method, an extrusion-type coating head first approaches a web, and then a coating liquid is extruded through a slit of the coating head onto the web to form a coating film, while the coating head and the web are spaced from each other with a clearance corresponding to a thickness of a film to be coated on the web. On coating the web by using said extrusion-type coating head, controlling the thickness of the coating film is unstable at the start of the coating wherein the coating head and the web move closer to each other, or at the end of the coating wherein the coating head and the web move away from each other. At these times, or at the unstable control times, the coating film is liable to become partially thicker, and thus a web feeding guide roll, etc., may be contaminated with the under-drying coating liquid of the thicker portion of the coating film on the web. Further, when magnetic coating liquid is used as a coating liquid for the manufacture of magnetic tapes or magnetic floppy discs, problems such as the occurrence of drop-out in products, reduction in yield, etc. have come up.
In order to prevent a coating film from becoming partially thicker in the extrusion coating method, the following various coating methods have already been proposed. For example, Japanese Laid-open Patent Application No. 2-229572 discloses a method wherein a thickness of a coating film is controlled with a solid smoother disposed at the downstream side of the coating head. Japanese Laid-open Patent Application No. 6-99123 discloses a coating method wherein a thicker portion of a coating film is scraped off and removed with a scraping roll disposed at the downstream side of a coating head. Japanese Laid-open Patent Application No. 62-95169 discloses a method wherein, at the start of coating a web, an extrusion of a coating liquid is started after a coating head has contacted with a web, and at the end of the coating, the extrusion of the coating liquid is stopped and after that the coating head moves away from the web, thereby preventing the coating film from becoming partially thicker. Further, Japanese Laid-open Patent Application No. 3-296467 discloses a method wherein an extrusion of a coating liquid begun after a back edge (it means an edge at the upstream side in the web feeding direction in this publication) of a coating head is approached to or slidably contacted with the web, and then a doctor edge (it means an edge at the downstream side in the web feeding direction in this publication) of the coating head is contacted with the web to prevent streaking on the coating film with chips which are scraped from the web on the edge of the coating head at the start of the coating.
However, in a double-side coating process using the extrusion coating head such as in the case where a coating film is formed on each of both sides of the web, that is, a first surface and a second surface, or an opposite side of said first surface, or in the case where a coating film is first formed on the first surface of the web by a coating method other than the extrusion coating method, and then a coating film is formed on the second surface of the web by using the extrusion coating method, the web cannot be supported by a support roll from the first surface side as the first surface per se has already been coated with a coating film. Hence, the occurrence of an increase in the thickness of the coating film on the second surface cannot be effectively prevented by using the smoother or the scraping roll as is disclosed in the Japanese Laid-open Patent Application Nos. 2-229572 and 6-99123, as said solid smoother or the scraping roll cannot be suficiently forced against the web because the web cannot be supported by the support roll from the opposite side of the film, and therefore the thicker portion of the film cannot be scraped off nor removed effectively by them.
In the method as disclosed in the Japanese Laid-open Patent Application No. 62-95169, at the start and the end of the coating, a coating head in a state of not extruding coating liquid is in contact with a running web in a solvent-vapor (volatile component) atmosphere of the coating liquid. Therefore, there is a danger that could cause an explosion and a problem that wear of the edge portion of the coating head accelerates.
A method disclosed in the Japanese Laid-open Patent Application NO. 3-296467 does not aim to prevent the increase in the thickness of a coating film at the start of the coating, and therefore no sufficient effects to prevent the increase in the thickness of a coating film are obtained. It also has no disclosure regarding the prevention of occurrence of the thicker portion when the coating head is separated from the web at the end of the coating.